In doing the project "No Pain, Lots of Game" which measures the ability of playing video games to distract the player from pain, do the video games played by the volunteers need to be the same? For example, can one volunteer play Mario Kart while another plays Halo 4, or is that too much of a variable? We know that it is a variable, but we also know that it's important for the volunteer to be engaged in the game. One game may be more engaging to some and not as much to another.

This sounds like a great project. You are correct that you need to keep your controlled variables the same. I think you should have all the test subjects play the same game. If you are worried about variability between different people try repeating the experiment on as many different people as possible.

Here is what it says about controlled variables:Experiments also have controlled variables. Controlled variables are quantities that a scientist wants to remain constant, and he must observe them as carefully as the dependent variables. For example, if we want to measure how much water flow increases when we open a faucet, it is important to make sure that the water pressure (the controlled variable) is held constant. That's because both the water pressure and the opening of a faucet have an impact on how much water flows. If we change both of them at the same time, we can't be sure how much of the change in water flow is because of the faucet opening and how much because of the water pressure. In other words, it would not be a fair test. Most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."

Always keep in mind that controlled variables are variables that stay the same.

It's a good idea to name your experiment specifically. Just using "video games" in your hypothesis is too general if you are only testing one certain video game. However, if you are testing more than one video game, it would probably be a good idea to list out the video game names in your hypothesis. If there are a lot of them being tested, "various video games" would also be acceptable. Be sure to test them in groups so the results don't get mixed up with other video games.

If you are testing multiple games, your hypothesis should be something like, "If various video games are tested for their ability to distract players from pain, then Halo 4 will have the highest ability." This hypothesis is a bit vague because you are not stating how you are going to test it. A bit more detail about what you are planning would be helpful. If you go with multiple video games, it helps you compare the video games in their abilities. If you are using one video game, it only measures the effect on pain for one game.

Keep in mind to use measurable things, like time in specific units (minutes, seconds, etc).